1. Field of the Invention
The present invention relates to an apparatus for detecting the position of a focusing lens (group) in an auto-focus surveying instrument, such as a total station.
2. Description of the Related Art
In a conventional auto-focus type surveying instrument having a collimating telescope, such as a total station, a light path of a focus detection optical system is split from a light path of the collimating telescope by means of a beam splitting optical system, wherein the focus state on a plane optically equivalent to a focal plane of the telescopic optical system is detected by a phase difference automatic-focus detector having a pair of CCD sensors to calculate the amount of defocus, so that the focusing lens group can be moved to an in-focus position based on the defocus amount. The principle of the phase difference AF device is well known and has been employed in AF single lens reflex cameras.
However, depending on the object (targeted object) by which the collimating operation is being applied, some automatic focus detectors cannot perform a precise auto-focusing operation. For example, if the targeted object is a prism which is used for a distance measuring operation, the auto-focus surveying instrument is focused on its own image formed on the prism, the position thereof determined as being further away from the actual position of the prism. To prevent this, it is theoretically possible to carry out the automatic focusing operation of the collimating telescope based on measurements (object distance values) obtained using a distance measuring function of the surveying instrument. In this solution, the distance of the targeted object must be correlated with the position of the focusing lens group at which the targeted object is in focus. Accordingly, it is necessary to provide a detection mechanism which detects an absolute position of the focusing lens group.
Moreover, when putting up a pole at a predetermined position in a pole staking operation, it is advisable to focus the optical system of the collimating telescope at a preset position. Accordingly, likewise with the above mentioned situation, the focusing lens group position detection mechanism is necessary.
However, in the detection of the position of the focusing lens group, an encoder which is directly connected to a focusing lens group driving motor detects the amount of rotation of the driving motor, so that the displacement of the focusing lens group can be determined in accordance with the amount of rotation thus obtained. Therefore, if the direction of the movement of the focusing lens group is reversed, detection of the absolute position of the focusing lens group cannot be carried out due to backlash of a driving gear train which transmits the rotation of the driving motor to the focusing lens group. Therefore, to move the focusing lens group to an in-focus position, it is always necessary to pass the focusing lens group through a predetermined reference position and move the same from the reference position without reversing. Such movement of the focusing lens requires a long time for the focusing operation, thus resulting in a reduced operation efficiency.
It is theoretically possible to detect the position of the focusing lens group without having influence from backlash which occurs in the driving gear train, using an optical linear encoder. However, an optical linear encoder is large and expensive.